The Role of WRKY Transcription Factors in Plant Immunity

Abstract

The WRKY TF superfamily consists of 74 and 109 members in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), respectively (Eulgem and Somssich, 2007; Ross et al., 2007). Members of this family contain at least one conserved DNA-binding region, designated the WRKY domain, comprising the highly conserved WRKYGQK peptide sequence and a zinc finger motif (CX_4–7CX_22–23HXH/C). This domain generally binds to the DNA element termed the W box (C/TTGACT/C), although alternative binding sites have been identified (Sun et al., 2003; Cai et al., 2008; Ciolkowski et al., 2008; van Verk et al., 2008). WRKY family members are divided into three groups based on the number of WRKY domains and certain features of the zinc finger-like motifs (Eulgem et al., 2000). The NMR solution structure revealed that the C-terminal WRKY domain of Arabidopsis WRKY4 consists of a four-stranded β-sheet, with a zinc-binding pocket formed by the conserved Cys/His residues located at one end of the β-sheet, and the WRKYGQK residues, corresponding to the most N-terminal β-strand (strand β-1), kinked in the middle of the sequence by the Gly residue (Yamasaki et al., 2005). The concave curvature of strand β-1 induced by this kink is predicted to enable this strand to deeply enter the DNA groove and make contact with bases of the W box element. The crystal structure of the extended WRKY domain of Arabidopsis WRKY1 (AtWRKY1-C) revealed that this domain is composed of a globular structure with five β-strands forming an antiparallel β-sheet with an additional novel zinc-binding site at one end (Duan et al., 2007). One should note, however, that no crystal structure information exists of a WRKY domain associated with its DNA-binding site or for a full-length WRKY protein.